The present invention concerns meters, such as those for measuring flow and fluid level, that make measurements by sending acoustic signals through fluids. It particularly concerns meters that are to be employed in explosion-hazard areas.
A typical way to measure a fluid's flow rate--either volume flow rate or velocity--is to send acoustic energy between two locations through the fluid, measure the delay between transmission from the one location and arrival at the other, repeat the operation in the reverse direction, and infer the flow rate from the difference. Another approach is to infer the flow rate from the correlation between the backscatter records that result from two successive acoustic-energy pulses. In both cases, the measurements commonly involve lossy fluids and noisy environments, so it is usually preferable to employ a relatively high-amplitude acoustic signal for this purpose.
But the ability to provide such high-amplitude signals is frequently limited by the possibility that explosive gases will be present, either at the location of the fluid whose flow is to be measured or in the path between that location and a monitoring location, where the flow-rate indication is displayed or recorded. In the presence of such gases, sparks resulting from signal-conductor faults can cause explosions.
Flowmeter suppliers have addressed this problem by so designing the system that any high voltages that occur as a result of a failure in the typically safe monitoring location cannot propagate from the safe region to the hazardous region along the signal lines that connect them. But the resultant low power that the remotely located transducers receive tends to keep the acoustic-signal amplitude low and thereby make the measurement relatively vulnerable to noise.
Some systems reduce noise effects by combining the numerous transmissions' records to produce a single measurement; the noise in successive records tends to cancel, while the desired signals reinforce each other. Still, the lower the acoustic-signal amplitude, the greater the number of such records that need to be added together, and the longer the time required for a single measurement. Particularly when the fluid flow is turbulent, moreover, the additional accuracy that results from adding together more records quickly diminishes. Indeed, there comes a point at which adding more records is actually counterproductive; if the time between the first and last transmissions is too great, the signal components of interest change enough that they no longer reinforce each other.
One way to avoid this problem is to use explosion-proof conduit to encase the cables that carry signals between the monitoring and monitored locations. But the cost and difficulty of working with the resultant cable make this approach too expensive for many applications.
One might propose instead to use an expedient that is employed in some other applications. If the transducer itself is not in a hazardous location, or if it can be provided with some type of safe container, energy supplied from the cable at low voltage levels can be stored up over a period of time and then concentrated into a high-power pulse. But even without the need for a safe container, the energy-storage circuitry required by such an approach greatly increases the expense of the apparatus at the transducer location, where only relatively inexpensive transducers need to be provided in non-hazardous installations. And the improvement would likely be illusory, anyway, since the time required to store the necessary energy would reduce the number of signal records that could be taken before the signal components of interest change too much to reinforce each other.